Television transmitting system



latented Oct. 7, 1941 UNITED STATES PATENT OFFICE William SpencerPercival, Ealing, London, Cecil Oswald Browne,-West Acton, London,Leonard Reginald Joffre Johnson, Chiswick, London, and Frank Blythen,Hayes, England, assignors to Electric 81, Musical Industries Limited,Hayes, Middlesex, England, a company of Great Britain ApplicationJanuary 18, 1938, Serial No. 185,480

In Great 12 Claims.

7 This invention relates to television transmitting systemsand isdirected to the reduction or elimination of spurious or undesiredsignals of irregular wave form, and of' relatively low frequency.

The invention is particularly concerned with transmitting systemsembodying a cathode ray tube picture scanner in which a screen structureis provided composed of a plurality of photosensitive mutually insulatedelements capacitatively associated with a signal plate, the elementsacquiring an electrostatic charge according to the intensity of thecorresponding elementary areas of the picture to be transmitted. Theelements Britain January 20, 1937 mosaic screen. In this case themosaic; screen may be composed of a sheet of material having a hightransverse resistance such as a sheet or mica. 7

It is the chief object of the present invention to provide an improvedtelevision transmission system in which the eliect of tilt can be morereadily overcome. i It is found by projectingor generating in thevicinity of the mosaic screen relatively slow speed electrons, that itis possible to stabilise the tilt component, thus permitting a correc-ting signal to be employed which may be kept more constant despite rapidchanges in contrast are-restored to an equilibrium potential under theof the picture being transmitted. action of a scanning beam of cathoderays, According to'the invention a method of genrestoration of theelements to the equilibrium cratingpicture signals for the transmissionof potential generating picture signals which are pictures to a distanceis provided wherein use fed to a suitable amplifier for transmission. ismade of a television transmission tube Dav- In such transmissionapparatus the spuriousor ing a mosaic screen upon which an image of theundesired signals give rise to what has become picture is projected, andin which picture sig known as tilt, and the tilting efiect is maninals'generat'edby such a tube suffer from the effested in the picturesignals by the signals having feet of tilt wherein, in order that acorrecting superimposed upon them a saw-tooth'like wave sign-al be usedwhich may be maintained or on a curved wave so that the picture signalsmore constant despite rapid" changes in contrast generated when scanninga line of constant of the images projected on to the mosaic screen,br'ishtncss'are such to correspond to a line having slow speedelectronsare generated within thea p o r ively h n ing b i h ne l n itstube in such a manner as to stabilise the tilt length. In a picture,reconstituted by means of a component. 4 cathode ray tube, the image onthe fluorescent Where a photo-sensitive mosaic screenis used screenappears to have a progressively .increasit is known that during theadmission of photo ing brightness towards one corner of thereconsensitive material to the tube during the manustitutedimage Inaddition ftilt is also manifacture thereof for photo-sensitising themosaic tested in that dark areas of the picture become screen, some ofthephoto-sensitive material bepartially filled with a white mist. comesdeposited on: other parts of the tube such- In order to reduce the cheatof tilt, it has as for example, the envelope, or on to what is beensuggested to apply an equal and opposite known as the second anode andother parts. saw-tooth or other wave form to counteract the With atubeoithis kind the slow speed electrons saw-tooth wave upon which thepicture signals for stabilising purposes may be generatedby are imposed.It is found that the tilt compoe0 projecting diffused? light eitherdirectly on to the nent varies according to the particular scene ormosaic screen (which thereby generate 1 picture being transmitted, andif the scene or speed'electrons) or on to other partsof the tube pictureor the light contrasts in the scene or piewhich are photo sensitive,particularly on to the ture change rapidly, then it is necessary to varysecond anode. The light which is projected the shapeoi the correctingsignals which, as will thereon generates the slow speed electrons necbeappreciated, give rise to complications in the essary for stabilizingpurposes. In the other adjustment of the signals and result in a verytype oitube. hereinbefore referred to, that is to complicated wave formfor the correcting sigsay theitube inwhich an opticalimage is pronals.jected: on to a photo-sensitive cathode and the Tilt is also found toarise in other types of photo-electron image emanating from thecathtelevision transmitting tubes such as a tube in ode is projected onto. a; substantially non-photo which the image of the picture or objectis prosensitive mosaic screen, the photo-sensitivemajected on to aphoto-sensitive cathode and the terial also becomes deposited on suchparts of photo-electron image from the cathode is pro- 7 the tube astheenvelope and the second anode j'ectcd om to a substantiallynon-photosensitive and inthis-case therefore slow speed electrons may begenerated by projecting diffused light on to photo-sensitive parts ofthe tube preferably in the vicinity of the mosaic screen. Alternativelythe slow speed electrons may be generated by employing one or morethermionic emitters, such emitter or emitters producing streams of slowspeed electrons which are directed towards or are present in thevicinity of the mosaic screen with a substantially uniform intensity.The speed of the electrons should be such as to be comparable with thespeed of photoelectrons.

In order that the said invention may be clearly understood and readilycarried into effect, it will now be more fully described with referenceto the accompanying drawing which illustrates diagrammatically a cathoderay, television transmitting tube suitable for use in the invention.

As shown, the tube comprises an evacuated envelope l in which is mountedthe mosaic screen 2 composed of a plurality of photo-sensitivemutually-insulated elements capacitatively associated with a signalplate 3. The picture to be transmitted is projected on to the screen 2through a suitable optical system indicated diagrammatically at 4. Onprojecting an image) on to the screen 2 the elements of the screenacquire electrostatic charges according to the intensity of thecorresponding elementary areas of the picture to be transmitted. Theelements of the screen are restored to an equilibrium potential underthe action of a scanning beam of cathode rays restoration of theelements to the equilibrium potential generating picture signals whichare fed to a suitable amplifier for transmission. The signal plate 3 isconnected to earth through a resistance 5 across which the picturesignals are developed, the picture signals being fed to the amplifier,not shown, from the arrow in the drawing. The scanning beam is generatedfrom a cathode 6 the electrons emanating from the cathode being focussedby an electron lens constituted by a first anode 1 and a second anode 8to which suitable potentials are applied in known manner. The cathoderay beam is deflected over the screen 2 by deflecting coils or plates,not shown. In a transmitting system embodying a transmitting tube asshown in the drawing, the picture signals fed to the amplifier will, ineffect, be superimposed upon a sawtooth-like or similar wave which givesrise to what is known as tilt.

During the photo-sensitising operation of the mosaic screen 2, otherparts of the tube such as the envelope I and the second anode 8, becomeslightly coated with the photo-sensitive material. In accordance withone feature of the invention light is projected on the screen 2 or on tothe second anode 8, or on to any other part or electrode of the tube inthe vicinity of the screen 2, or on to one or more of these elementswhich are photo-sensitive for the purpose of generating slow speedelectrons which it is found serve to stabilise the tilt component.

The light is preferably derived from a direct current source although insome cases light from an alternating current source, preferably of lowfrequency, may be employed for the samepurpose. As shown in the drawing,a light source 9 in the'form of filament is provided internally of theenvelope I, although in some cases the light source may be arrangedexternally 0f the tube. The intensity of the light projected on to themosaic screen 3 or other part will be determined empirically. The lightsource 9 may, if desired, be disposed in other positions in the envelopeI. It is however preferable to arrange the light source to the rear ofthe signal plate 3 or otherwise in such a manner that although themosaic screen 2 receives substantially no light directly from the lightsource, light is permitted to reach the second anode 8 for the purposeabove mentioned. In this construction the sensitivity of the mosaicscreen is increased and in addition the arrangement permits of a moreintense light to be employed. For example a 12 volt lamp consumingmilliamperes may be used. Since in accordance with the invention thetilt component is stabilised, the correcting signal which it isnecessary to superimpose on the signals derived from the transmittingtube, can be maintained substantially constant in form despite changesin the light contrasts of the object to be transmitted.

As stated above, it is possible that the light source 9 may be replacedby a source or sources generating slow speed electrons. In some cases anadditional electrode may be provided suitably disposed within the tubeand made photosensitive, the light source being disposed so that thelight rays are projected only on to the additional electrode.

The cathode ray tube shown in the drawing is of one particular kind inwhich an image of the object for transmission is projected directly onto the mosaic screen 2. The invention can also be employed inconjunction with transmitting tubes in which a scene or view from a filmor other transparency is projected on to the mosaic screen. In thelatter case it may be arranged that the transparency is movedintermittently so that the picture is projected on to the mosaic screenalso intermittently or it may be arranged that the transparency is movedcontinuously and a mirror system or other arrange ment interposedbetween the transparency and the mosaic screen so that the picture is ineffect projected on to the mosaic screen intermittently.

The invention may also be applied in connection with other types oftransmitting tubes in which tilt occurs.

In these cases, and where the invention is employed for the transmissionof pictures in other manners, the light or the emission of the slowspeed electrons may occur continuously or intermittently during theintervals between successive frames of the picture or transparency.

We claim:

1. The method of operating a light sensitive tube wherein is provided aphotosensitive mosaic electrode and a further photoelectric surface,which includes the steps of continuously photoelectrically releasingelectrons of substantially constant number and velocity from saidsurface, and flooding the photosensitive mosaic electrode with thereleased electrons. 2. The method of operating a light sensitive tubewherein is provided a photosensitive mosaic electrode and a furtherphotoelectric surface, which includes the steps of producingincandescent light, continuously photoelectrically releasing electronsof substantially constant number and velocity by the produced light fromsaid surface, and flooding the photosensitive mosaic electrode with thereleased electrons.

3. The method of operating a light sensitive tube wherein is provided aphotosensitive mosaic electrode and. a further photoelectric surfaceelectrode,

which includes the steps of projecting a light image upon the mosaicelectrode, continuously photoelectrically releasing electrons ofsubstantially constant number and velocity within the tube from saidsurface, and flooding the mosaic electrode with the released electrons.

4. In a cathode ray tube wherein is positioned a photosensitive mosaicelectrode and an electron gun having an anode having a photo-sensitizedsurface, the method of operation which comprises the steps of projectinga light image on the mosaic electrode, continuously photoelectricallyreleasing electrons of substantially constant numher and velocity fromthe said surface of said anode, flooding the mosaic electrode with thereleased electrons, and scanning the mosaic electrode by electrons fromthe electron gun.

5. An electron system comprising an evacuated envelope, a photosensitivemosaic electrode positioned within the envelope, a photosensitizedsurface and an electron gun having an anode for developing a beam ofelectrons to scan the mosaic positioned within the envelope, means tocontinuously photoelectrically release electrons of substantiallyconstant number and velocity Within the envelope from said surface, andmeans to flood the mosaic electrode with the released electrons.

6. A light translating system comprising a photosensitive mosaicelectrode, a photoelectric surface adjacent said electrode means toproject an optical image upon the mosaic electrode, light means tocontinuously photoelectrically release electrons of substantiallyconstant number and velocity from said surface, and means to flood themosaic electrode with the released electrons.

7. A cathode ray tube comprising a photosensitive mosaic electrode, aphotosensitive surface adjacent said electrode, an electron gun, meansincluding an anode for directing a beam of electrons from the electrongun toward the mosaic a source of radiant energy positioned within thetube for continuously photoelectrically releasing electrons ofsubstantially constant number and velocity from said surface, and asignal plate mounted integral with the mosaic electrode.

8. In a television system wherein is provided a cathode ray tube havinga photosensitive mosaic electrode and an electron gun having an anodehaving a photosensitized surface, the method of producing signalsrepresentative of a light image to be transmitted which includes thesteps of projecting a light image on the mosaic electrode, continuouslyphotoelectrically releasing electrons of substantially constant numberand velocity from the photosensitized surface of said anode, floodingthe mosaic electrode with the released electrons, scanning the mosaicelectrode by electrons from the electron gun, and converting thevariation in electric charges resulting from the scanning operation intoelectrical potentials representative of the change of electric charge.

9. A method of operating a light sensitive tube wherein is provided aphotoelectric surface and a mosaic electrode having a photosensitivearea facing the photoelectric surface which includes the steps ofcontinuously projecting light from behind the photoelectric area of themosaic upon the photoelectric surface to release electrons therefrom andimpacting the released electrons on the photosensitive area.

10. The method of operating a cathode ray tube wherein is provided aphotosensitive mosaic electrode and a further photoelectric surface inclose proximity to said mosaic which includes the steps of illuminatingthe photosensitive mosaic with an optical image and photoelectricallymaintaining the photoelectric surface at a substantially predeterminedconstant equilibrium potential relative to the mosaic electrode byprojecting light upon the photoelectric surface.

11. The method of operating a cathode ray tube wherein is provided aphotosensitive mosaic electrode and a further photoelectric surface inclose proximity to said mosaic which includes the steps of projectinglight continuously on the photoelectric surface to releasephotoelectrons therefrom and maintaining the photoelectric surface at asubstantially predetermined constant equilibrium potential with respectto the mosaic electrode by collecting the released photoelectrons.

12. A system for producing electrical image signal energy whichcomprises a cathode ray tube having included therein a photosensitivemosaic electrode, means for producing a cathode ray beam for scanningthe photosensitive mosaic electrode, and a photoelectric surfacecoating, means to project an optical image upon the mosaic electrode todevelop thereupon electrostatic charges of magnitudes normallyproportional to image brightness, means to scan the mosaic electrode bythe produced scanning beam to translate the charges into signal energyand means to project continuously radiant energy upon the photoelectricsurface to release photoelectrons for compensating for the spuriouscharges developed upon the mosaic electrode during the translation ofthe said electrostatic charges into signal energy.

WILLIAM SPENCER PERCIVAL.

CECIL OSWALD BROWNE.

LEONARD REGINALD JOFFRE JOHNSON. FRANK BLYTHEN.

